Plants make their food themselves with their leaves, and this is the food that is used for root growth. So cutting back the leaves more likely reduces root growth, it doesn't send energy to the roots. The reason we cut back when transplanting bare-root is to reduce transpiration water loss which can be life-threatening. When you dig up a plant it will almost inevitably lose some roots. Those roots are what supply the top growth with water which passes up through the plant and out of the stomata (pores) in the leaves - this is transpiration and it's a cooling mechanism for the plant. If you leave all the leaves on then the plant is out of balance because the root area for supplying those leaves with water has been reduced.
If the leaves wilt as a result of the water shortage, then the stomata will close and the plant is no longer making food (photosynthesizing) anyway, so it is a lose-lose situation. Hence we reduce the leaf area so that the remaining roots can supply enough water for the tops. There are times of year when you may get away with not cutting back for transplanting - transpiration rates are low during cool, cloudy, humid weather for example. But if the plant starts to wilt, you've lost the food producing capability of the leaves and you are better to cut them back so that the roots don't have as much leaf to try and supply with water.
